System and method for pasteurizing citrus juice using microwave energy

ABSTRACT

A system and method for pasteurizing citrus juice with the use of microwave energy provide a plurality of microwave chambers through which juice is continuously flowed, the sequential flowing permitting a gradual increase in juice temperature that is sufficient to pasteurize the juice but insufficient to cause a detectable loss of flavor.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08/322,828,"System and Method for Pasteurizing Citrus Juice Using MicrowaveEnergy," filed Oct. 13, 1994, now U.S. Pat. No. 5,514,389.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for treatingliquids and, more particularly, to systems and methods for pasteurizingcitrus juice.

2. Description of Related Art It has been known since the days ofPasteur that, in order to prevent contamination over time by harmfulmicroorganisms, liquids can be heat treated. In addition, in fruitjuices, the enzyme pectin methylesterase (PME) causes a loss of qualityin the product during storage. Commercial steam pasteurization istypically performed by passing the product through a metal heatexchanger; however, degradation of the juice can occur if care is nottaken to minimize the temperature differential between the metal wallsof the heat exchanger and the center of the liquid.

Various systems and methods of achieving pasteurization have beendisclosed, including those of Huzenlaub (U.S. Pat. No. 2,517,569) andGray (U.S. Pat. No. 3,092,503), who teach the utilization of infraredheating for sterilization. Similarly, electromagnetic waves may be usedto heat a liquid, as discussed by Shaw (U.S. Pat. No. 2,585,970), whoteaches the use of waves of frequency 500-5000 MHz to excite aconductive cavity through which fluid is directed.

Microwave heating has also been proposed to inactivate enzymes andmicroorganisms in fruit juices by Copson (U.S. Pat. No. 2,833,657) andby Le Viet (U.S. Pat. No. 4,694,133). In the latter of these, a swirlingof the liquid is induced to achieve more uniform heating. The use ofmicrowave energy avoids some of the problems associated with heating tohigh temperatures, including loss of flavor. This is due in part to thefact that the radiant energy heats the juice directly without heatingthe chamber walls, thus avoiding heat-transfer films and permittingimproved temperature control. In addition, less heat is lost to theenvironment, and rapid startup and shutdown are possible.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved systemand method for pasteurizing liquids with the use of microwave energy.

It is an additional object to provide a system and method forpasteurizing liquids that avoids large temperature differentials duringthe process.

It is a further object to provide a system and method for pasteurizingliquids that improves the effective storage time of the liquid.

It is another object of the invention to provide a system and method forpasteurizing citrus juices that avoids loss of flavor overfresh-squeezed juice.

It is yet a further object to provide a system and method forpasteurizing citrus juices that inactivates enzymes and destroysmicroorganisms during processing.

These and other objects are attained with the use of the presentinvention, comprising a system and method for pasteurizing a liquidwhile retaining optimum flavor. Successful pilot tests were performed onorange juice that proved the efficacy of utilizing a single microwavechamber through which the liquid to be processed was continuouslyflowed. In these studies, it was shown that bacteria and PME could bereduced in concentration to levels permitting increased shelf life ofthe juice over nonpasteurized product. Taste tests indicated that therewas no measurable loss of flavor over fresh-squeezed juice, an importantcriterion for a liquid to be commercially viable.

In order to improve cost effectiveness, ease of maintenance, andflexibility, a modular system was designed to utilize a plurality ofmicrowave chambers. The method of the present invention thus comprisesthe steps of continuously flowing the liquid sequentially through aplurality of chambers and heating the interiors of the chambers withmicrowave energy to raise the temperature of the liquid to apredetermined final temperature. The total microwave power and residenttime in the chambers are designed to be sufficient to pasteurize theliquid, and the rate of temperature increase in the liquid through thechambers are insufficient to cause a detectable loss of flavor in theliquid. A further step, which improves the efficiency of the method,comprises exchanging heat between liquid entering the system and liquidexiting the system. The energy efficiency is attained by recapturing alarge portion of the energy that had been used to heat the exiting fluidand imparting some of that heat to the entering fluid, thus requiringless of the microwave power.

In a preferred embodiment, the flowing step comprises flowing the liquidthrough tubing substantially transparent to microwave energy. Also in apreferred embodiment, the tubing within each chamber is formed intocoils. This increases the potential volume of liquid within each chamberwhile simultaneously maintaining a maximum depth of liquid from thetubing wall. This technique minimizes the temperature gradient that canform in the liquid being heated, which is preferable to subjecting alarge volume of liquid to microwave energy, wherein the liquid can havesubstantial temperature variations throughout the volume.

In a specific embodiment, the material of the tubing used comprisespolypropylene.

The system of the present invention for pasteurizing a liquid whileretaining optimum flavor comprises a plurality of chambers, each chamberhaving an interior, and means for continuously flowing the liquidsequentially through the chambers. In addition, the system comprisesmeans for heating the interiors of the chambers with microwave energy toraise the temperature of the liquid to a predetermined finaltemperature. Also provided are means for controlling the total microwavepower emitted by the heating means and means for controlling the flowingmeans to control the resident time of the liquid in the chambers. As forthe method described above, the system is configured to that themicrowave power and resident time are sufficient to pasteurize theliquid, and the subsequent rate of temperature increase in the liquidcaused by the power and resident time within the chambers areinsufficient to cause a detectable loss of flavor in the liquid.

In a preferred embodiment, the plurality of chambers comprises a firstchamber and a final chamber, and the system further comprisesheat-exchange means for exchanging heat between liquid entering thefirst chamber and liquid exiting the final chamber. The heat-exchangemeans is utilized to improve the efficiency of the system.

The features that characterize the invention, both as to organizationand method of operation, together with further objects and advantagesthereof, will be better understood from the following description usedin conjunction with the accompanying drawing. It is to be expresslyunderstood that the drawing is for the purpose of illustration anddescription and is not intended as a definition of the limits of theinvention. These and other objects attained, and advantages offered, bythe present invention will become more fully apparent as the descriptionthat now follows is read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an arrangement of an embodiment of the system andmethod for pasteurizing and disinfecting citrus juice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description of the preferred embodiments of the present invention willnow be presented with reference to FIG. 1, wherein the system isdesignated by the reference numeral 10.

A preferred embodiment of the system and method disclosed hereincomprises a system and method for pasteurizing, which inactivatesunwanted enzymes, and disinfecting, to destroy microorganisms citrusjuice 50 while retaining optimum flavor. The specific embodiment to beoutlined is a system and method for pasteurizing orange juice, althoughthis is not intended to limit the scope of the invention.

The steps of the method comprise continuously flowing the juice 50through the system 10 of the present invention, the system 10 comprisingat least six chambers 101-106 connected in series, each having aninterior space 191-196, respectively. The first chamber is designated101, and the final chamber, 106. In the embodiment tested, eachmicrowave chamber comprised a Questron continuous-wave magnetrongenerating useful power output of 1000 W at 2450 MHz. A commercial-sizeunit requires larger units, typically 5000 W. The power emitted by eachchamber 101-106 is also controllable via power controller 190.

The continuous flow is effected with the use of a pump 40, driven by avariable-speed motor 42. In the embodiment tested, the pump 40 iscapable of pumping juice at 60 gallons per hour; however, commercialunits typically pump at ten to a hundred times that rate, or 600-6000gallons per hour. Additional control is attained with inlet 112 andoutlet 114 valves fitted to each of the chambers 101-106 and shown inFIG. 1 on chamber 102. Each unit may also be activated/deactivated withon/off switch 116.

The juice 50 is pumped through the system 10 through tubing 20substantially transparent to microwave energy, in a preferred embodimentcomprising polypropylene having fiberglass cladding for insulation. Inorder to increase the potential volume of juice within each chamberwhile simultaneously maintaining a maximum depth of juice from thetubing wall 204, the tubing 20 within each chamber 101-106 is formedinto coils 202. In the preferred embodiment, the tubing has an innerdiameter of 3/8 inch, this diameter being selected to ensure that themicrowave energy absorption by the liquid is less than 50%.

The interiors 191-196 of the chambers 101-106 are heated with microwaveenergy, in the preferred embodiment having a frequency of 2450megahertz.

The temperature of the juice in the final chamber 106 is sensed with asensor 32. Another sensor 30 senses the temperature of the enteringjuice at the entrance 108 of the system. In response to the sensedexiting temperature, the flow of the juice through the system iscontrolled in order to attain the desired final temperature of juice 50in the final chamber 106. This desired final juice temperature isdetermined that is sufficiently high to inactivate undesirablemicroorganisms and enzymes, including bacteria and pectinmethylesterase, and at the same time sufficiently low to avoid causingsubstantial deterioration of flavor. For the system and method fortreating orange juice, the desired final temperature is in the range of70 and 95 degrees centigrade. Specifically, for the preferredembodiment, the desired final temperature is in the range of 90 to 95degrees centigrade.

In order to improve the efficiency of the system and method, a furtherstep comprises exchanging heat between juice 502 entering the system 10at the entrance 108 and juice 504 exiting the system 10 at exit 110.Specifically, if exiting juice 504 at a temperature in the range of 90to 95 degrees centigrade is used to preheat entering juice 502 throughthe use of a heat exchanger such as a plate heat exchanger 60, themicrowave power requirement is decreased from 30 to 6 kilowatts,indicating that 80% of the energy can be recaptured and used to preheatentering juice 502 without overheating it.

Following the heat-exchange step, the exiting juice 504 is chilledwithin cooler 65 to a predetermined storage temperature to improve theeffective storage time of the juice 506. In the preferred embodiment,the predetermined storage temperature comprises a temperature ofsubstantially 4 degrees centigrade.

The method and system of the present invention have been used tocontinuously pasteurize orange juice, the treated juice then beingsubjected to flavor and microbial activity tests. At a final exitingtemperature of 90 degrees centigrade and a residence time of 8 seconds,PME was inactivated 100%, and bacteria were inactivated to less than thedetection limit of less than 200 colony-forming units per milliliterwhen subjected to agar incubation at 30 degrees centigrade for 48 hours.Taste panel testing has indicated that the microwave treatment does notcause a detectable change in flavor over fresh-squeezed unpasteurizedjuice at the 99.9% significance level. No significant deterioration inflavor was noted for 90 days when the juice is maintained at 4 degreescentigrade following treatment.

It may be appreciated by one skilled in the art that additionalembodiments may be contemplated, including the pasteurization anddisinfection of milk, beer, and other fruit juices.

In the foregoing description, certain terms have been used for brevity,clarity, and understanding, but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchwords are used for description purposes herein and are intended to bebroadly construed. Moreover, the embodiments of the system and methodillustrated and described herein are by way of example, and the scope ofthe invention is not limited to the exact details of the system andsteps outlined above.

Having now described the invention, the construction, the operation anduse of preferred embodiment thereof, and the advantageous new and usefulresults obtained thereby, the new and useful constructions, andreasonable equivalents thereof obvious to those skilled in the art, areset forth in the appended claims.

What is claimed is:
 1. A method for pasteurizing a liquid whileretaining optimum flavor, the method comprising the stepsof:continuously flowing the liquid through microwave-transparent tubing,the tubing having a diameter sufficient to minimize a temperaturegradient thereacross by maintaining a maximum depth of liquid from awall of the tubing; and exposing the liquid flowing through the tubingto microwave energy in a series of steps to sequentially heat theflowing liquid to a predetermined final temperature sufficiently high topasteurize the liquid but insufficient to cause a detectable loss offlavor in the liquid.
 2. The method recited in claim 1, furthercomprising the step of exchanging heat between liquid about to beexposed to the microwave energy and liquid that has been heated to thepredetermined final temperature in order to preheat entering liquid andthereby improve an efficiency of the method.
 3. The method recited inclaim 1, wherein the flowing step comprises flowing the liquid throughcoiled tubing.
 4. The method recited in claim 1, further comprising thesteps of:sensing a temperature of the heated liquid; and controlling theflow of the liquid through the microwave energy field in response to thesensed temperature in order to attain the desired final temperature. 5.A method for pasteurizing citrus juice while retaining optimum flavor,the method comprising the steps of.continuously flowing the juicethrough microwave-transparent tubing, the tubing having a diametersufficient to minimize a temperature gradient thereacross by maintaininga maximum depth of juice from a wall of the tubing; and exposing thejuice flowing through the tubing to microwave energy in a series ofsteps to sequentially heat the flowing juice to a predetermined finaltemperature in the range of 70 to 95 degrees centigrade, the finaltemperature sufficiently high to pasteurize the juice but insufficientto cause a detectable loss of flavor in the juice.
 6. A system forpasteurizing a liquid while retaining optimum flavor, the systemcomprising:a plurality of sources of microwave energy for heating theliquid; means for continuously flowing the liquid so as to expose theliquid sequentially to the microwave energy sources, thereby raising atemperature of the liquid sequentially; means for controlling a totalmicrowave power emitted and the flowing means to control a time theliquid is exposed to microwave energy, the exposure time sufficient topasteurize the liquid and insufficient to cause a detectable loss offlavor in the liquid.
 7. The system recited in claim 6, wherein theflowing means comprises:tubing substantially transparent to microwaveenergy; and means for pumping the the liquid through the tubing.
 8. Asystem for pasteurizing citrus juice while retaining optimum flavor, thesystem comprising:a plurality of sources of microwave energy for heatingthe juice; means for continuously flowing the juice between a juiceinlet and a juice outlet so as to expose the juice sequentially to themicrowave energy sources, thereby raising a temperature of the juicesequentially; means for controlling a total microwave power emitted andthe flowing means to control a time the juice is exposed to microwaveenergy, the exposure time sufficient to pasteurize the juice andinsufficient to cause a detectable loss of flavor in the juice, theexposure time further sufficient to raise the juice temperature to afinal temperature sufficient to inactivate undesirable microorganismsand enzymes but insufficient to cause a substantial deterioration offlavor.
 9. The system recited in claim 8, further comprising:means forsensing the final temperature of the juice, the sensing means positionedadjacent the outlet of the system; and means for controlling the flow ofthe juice through the system in response to the sensed temperature inorder to attain the final temperature.
 10. The system recited in claim8, further comprising means for exchanging heat between juice enteringthe system and juice exiting the system, for preheating the enteringjuice and for improving an efficiency of the system.